[ODS] Introduce IntEnumAttr

In ODS, right now we use StringAttrs to emulate enum attributes. It is
suboptimal if the op actually can and wants to store the enum as a
single integer value; we are paying extra cost on storing and comparing
the attribute value.

This CL introduces a new enum attribute subclass that are backed by
IntegerAttr. The downside with IntegerAttr-backed enum attributes is
that the assembly form now uses integer values, which is less obvious
than the StringAttr-backed ones. However, that can be remedied by
defining custom assembly form with the help of the conversion utility
functions generated via EnumsGen.

Choices are given to the dialect writers to decide which one to use for
their enum attributes.

PiperOrigin-RevId: 255935542